This invention relates to arc discharge lamps, and more particularly, to a discharge lamp having an improved mount for coaxially supporting an arc tube within an outer envelope in a manner which is self-centering and provides thermal expansion compensation.
The present invention is particularly useful in the mounting of arc tubes in high pressure sodium vapor arc discharge lamps. Such lamps employ a relatively long tubular arc tube of narrow diameter and composed of polycrystalline alumina. Internal electrodes at each end of the arc tube are respectively connected to external terminals comprising niobium feed-through tubes. U.S. Pat. Nos. 3,623,134, 3,996,487 and 4,037,129 illustrate prior methods of coaxially supporting the arc tube within the outer envelope by means including a rather heavy metal frame member having spring fingers disposed at one or both ends of the lamp for resiliently engaging the inner wall of the outer envelope to aid in positioning the arc tube. In two of the aforementioned patents, the base-facing niobium tube terminal of the arc tube is supported on a rod or heavy conductor wire projecting from the base; in the other patent, the base-facing terminal of the arc tube is connected through a multi-segment arrangement including a flexible lead wire. In all three patents, the dome-facing terminal of the arc tube is connected to a rod or legs welded to the supporting frame member.
In another heavy frame type support arrangement which has been widely employed in high pressure sodium discharge lamps, the frame, and thus arc tube, is centered and anchored by means of a resilient clamp secured about an inwardly projecting dimple formed at the dome end of the outer envelope; U.S. Pat. No. 3,384,798 provides an example for such a mounting arrangment. Another arc tube mounting arrangement involving a dimple in the dome of the outer glass envelope is described in U.S. Pat. No. 4,117,371, wherein a comparatively heavy conductor wire extends beyond the length of the arc tube and has a loop formed on the end which is wrapped around the dome dimple.
A more simplified arc tube mounting arrangement is shown in U.S. Pat. No. 3,706,901 of De Neve, wherein a lead-in conductor from the base supports the base-facing arc tube terminal, while the dome-facing terminal of the arc tube is connected to the base lead-in wire by means of a terminal wire that includes a helically bent portion which extends along the straight part of the arc tube and a convolute portion which bears against the inner side of the outer envelope dome for centering the arc tube. The thickness of the terminal wire is disclosed as approximately 2 millimeters (about 0.08 inch); hence, the convolute portion, which De Neve shows as being hook-shaped, provides a rather rigid centering member. De Neve refers to the helically bent portion of the terminal wire as an anti-shadow winding, that will expand when heat is generated in the discharge tube, but due to its helical form, the expansion only causes the piece of wire to be bent out further, thereby avoiding the risk of cracks in the outer envelope.
The arc tube of polycrystalline alumina ceramic in a high pressure sodium vapor lamp may be heated in operation to average temperature of 1,000.degree. C. or higher, and the material has a coefficient of linear expansion of 8.times.10.sup.-6 per .degree.C. The arc tube is so hot that the only practical way of supporting it is by the inleads extending from the ends. In a lamp having an arc tube which is about 11 centimeters long, for example, the linear thermal expansion of the arc tube can be about 1 mellimeter. Accordingly, it has been found necessary to accommodate differential expansion in order to avoid undue stress on the end terminals and seals which may cause cracks in the alumina arc tube.
The prior art constructions, as described hereinabove, do not adequately allow for a thermal expansion of the arc tube, even though the base-facing niobium tube terminal is not welded to the base support rod. In actual practice on a production line, the arc tube is not perfectly centered, whereupon during thermal expansion, sufficient stress may often be applied to the arc tube whereby cracks are caused in the alumina. Previous approaches to this problem include U.S. Pat. No. 3,882,346, which describes a thermal expansion compensating mounting in which one end of the arc tube is rigidly fastened to a supporting frame, while the other end is secured by an axial lead wire which extends from a ceramic plug through a single close-fitting loop in a transverse support wire, and a flexible conductor provides the electrical connection. U.S. Pat. No. 4,254,355 describes a ceramic arc tube mounting in which the in-lead at one end of the arc tube is fastened to the support rod conductor, while the in-lead at the other end of the arc tube extends through an insulating bushing supported from the rod and connected by a curved flexible conductor to the other lead-in of the outer envelope. Differential thermal expansion is accommodated by sliding of the in-lead through the bushing and flexing of the curved conductor.
In addition to the aforementioned arc tube mounting considerations, it is not feasible, at the present state of the art, to accurately control the length of the alumina tube. Therefore, arc tubes for a given lamp type will vary considerably in length, thereby making it very difficult to provide proper mounting. Further, the presence of a frame wire of sufficient diameter and location to support the arc tube causes a noticeable shadow from the illuminated lamp.